1. Field of the Invention
This disclosure relates to an image forming apparatus configured to transfer a toner image on an image carrier by registering indices on a belt member and the image carrier.
2. Description of the Related Art
In the related art, a so-called tandem type image forming apparatus configured to overlap a second toner image formed on a second image carrier on a first toner image formed on a first image carrier and conveyed by a belt member, and transfer the overlapped toner images is widely used. In such an image forming apparatus, in order to avoid positional misalignment of overlap between the first toner image and the second toner image in a direction of conveyance, for example, JP-A-64-6981 describes a configuration in which registration marks formed out of an image area on a belt are detected by an imaging device, and the exposure start timing with respect to a photosensitive drum is adjusted in accordance with misalignment of the timing of detection.
In addition, JP-A-2009-134264 discloses a configuration in which a speed of rotation of a photosensitive drum on which an image is being formed (or a position of an intermediate transfer belt in the direction of rotation) is adjusted so as to register a second positional index magnetically recorded on the photosensitive drum with respect to a first positional index that is magnetically recorded on the intermediate transfer belt.
Furthermore, JP-A-2012-103649 discloses a configuration in which a speed of rotation of an image being formed is adjusted so as to register an electrostatic image index formed on an end of a second photosensitive drum with respect to an electrostatic image index transferred from a first photosensitive drum to an electrostatic image recording zone of an intermediate transfer belt.
Since the technology disclosed in JP-A-64-6981 is configured to adjust the exposure start timing, out-of-registration of color in short cycles caused by partial expansion or contraction of the belt being operated or speed variations is difficult to correct.
In contrast, the image forming apparatuses disclosed in JP-A-2009-134264 and JP-A-2012-103649 are configured to register the position of the second positional index on the image carrier with the position of the first positional index on the intermediate transfer belt by adjusting the speed of rotation of the photosensitive drum, so that the problem as described above rarely occurs. However, speed variations may occur in photosensitive drum when adjusting the speed of rotation of the photosensitive drum. When such speed variations are significant, turbulence of a scanning line pitch of an image may be generated and hence quality of an output image may be degraded.